FOSSIL PISHES. 5 



into a row of conical teeth, about half an inch in lengtli. In D. Terrelli, 

 on tlie contrary, the margin of the mandible here forms a sharp cutting 

 edge. At the posterior end of this edge it is sometimes obscurely crenu- 

 lated by what are evidently rudimentary teeth, the dwarfed and abortive 

 homologues of those wliich occupy the margins of the maxillaries and man- 

 dibles of D. Rertseri. 



The dentition of the upper jaw consists of what I have called, for con- 

 \euienee in^my description, premaxillaries and maxillaries, without, how- 

 ever, intending to commit myself fully to this view of their liomologies.* 

 Tliese have been partially described in the notice of Dinicthys contained 

 m Yol. I., Part II., of this Eeport, but new material has made it possible 

 to give a fuller description of them now. 



Beginning at the anterior extremity of the^head, the muzzle is termi- 

 nated by two large, triangular " premaxillaries," of which the upper sides 

 are flattened and concave, while the. opposite angles project downward to 

 form great, incisor-like teeth. These interlock with and shut over the 

 projecting points of the turned up mandibles, which are received into 

 their concavities. 



Behind the premaxillaries, two oblong dental plates or maxillm are set 

 directly over the prominent, denticulated, or cutting edges of the man- 

 dible. In D. Ilertzeri the mmoilloB are somewhat irregular in outline, 

 the lower margins being the longer and set with teeth similar to and inter- 

 locking with those of the mandible below. In JD. Terrelli the maxillaries 

 Kre oblong or quadrangular, with rounded angles, and the lower edges are 

 sharp and knife-like, and overlap and play upon the sharp edges of the 

 mandibles. 



When we compare this peculiar dentition with that of other fishes, we 

 find that Cocoosteus among fossil, and Zepidosiren among living fishes, 

 offer some remarkable and suggestive points of resemblance. 



The dentition of Coccosteus will be referred to further on in connection 

 with some other anatomical features which it has in common with Dinich- 



* In describing these bones I liave called them premaxillaries and maxillaries, because 

 they hold the positions and perform the functions of these organs in other fishes. We 

 have not yet found the anterior and upper portions of the head so well preserved that its 

 structure can all be made out, and we can only be certain of the homologies of the bones 

 in question when we shall have obtained more, and more perfect, material. One head oi 

 D. Ilertzeri, found at Delaware by Mr. Hertzer, shows the " premaxillaries " and " max- 

 illaries " in position, but the interior of the head is not visible. As will be shown further 

 on, the dentition of Dinichthya corresponds very closely with that of Lepidomren, but 

 even with full proof of identity of structure in the dentition of these genera, the ques- 

 tion before us would hardly be cleared of doubt, as what Professor Owen calls maxilla- 

 ries in Lepidosiren, Professor Huxley calls, probably with good reason, palato-pterygoid 

 dental plates. 



